Antenna Systems and Methods for massive MIMO Communication

ABSTRACT

Antenna systems and methods for Massive Multi-Input-Multi-Output (MIMO) (M-MIMO) communication are provided. Antennas systems include a M-MIMO transmitter architecture with a hybrid matrix structure. The hybrid matrix structure protects against transmit path component failures and ensures that a spatial rate of the MIMO transmitter is not degraded by the failures. Antenna systems and methods also include antenna selection schemes for selecting a subset of antennas from a plurality of antennas to transmit to a receiver.

CROSS-REFERENCE TO RELATED APPLICATION(S)

The present application claims the benefit of U.S. ProvisionalApplication No. 61/812,029, filed Apr. 15, 2013, which is incorporatedherein by reference in its entirety.

BACKGROUND

1. Technical Field

The present disclosure relates generally to antenna systems and methodsfor Massive Multi-Input-Multi-Output (MIMO) (M-MIMO) communication.

2. Background Art

In a Massive Multi-Input-Multi-Output (MIMO) (M-MIMO) communicationsystem, a transmitter, such as a base station, is equipped with a verylarge number of transmit antennas (e.g., 32, 64, or 100) that can beused simultaneously for transmission to a receiver, such as a userequipment (UE). The receiver can have more than one receive antenna(e.g., 2, 4, 8, etc.) for simultaneously receiving transmissions fromthe transmitter. The receiver can also be equipped with a very largenumber of receive antennas.

BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES

The accompanying drawings, which are incorporated herein and form a partof the specification, illustrate the present disclosure and, togetherwith the description, further serve to explain the principles of thedisclosure and to enable a person skilled in the pertinent art to makeand use the disclosure.

FIG. 1 illustrates an example Massive Multi-Input-Multi-Output (MIMO)(M-MIMO) transmitter.

FIG. 2 illustrates an example M-MIMO transmitter using a hybrid matrixaccording to an embodiment.

FIG. 3 illustrates an example hybrid matrix according to an embodiment.

FIG. 4 illustrates an example hybrid coupler that can be used in ahybrid matrix according to an embodiment.

FIG. 5 illustrates an example M-MIMO transmitter using an antennaselection matrix according to an embodiment.

FIG. 6 illustrates another example M-MIMO transmitter using an antennasselection matrix according to an embodiment.

FIG. 7 illustrates an example M-MIMO transmitter using an antennaselection matrix and a hybrid matrix according to an embodiment.

FIG. 8 illustrates an example process for M-MIMO antenna selectionaccording to an embodiment.

FIG. 9 illustrates another example process for M-MIMO antenna selectionaccording to an embodiment.

FIG. 10 illustrates another example process for M-MIMO antenna selectionaccording to an embodiment.

The present disclosure will be described with reference to theaccompanying drawings. Generally, the drawing in which an element firstappears is typically indicated by the leftmost digit(s) in thecorresponding reference number.

DETAILED DESCRIPTION OF EMBODIMENTS

For purposes of this discussion, the term “module” shall be understoodto include at least one of software, firmware, and hardware (such as oneor more circuits, microchips, processors, or devices, or any combinationthereof), and any combination thereof. In addition, it will beunderstood that each module can include one, or more than one, componentwithin an actual device, and each component that forms a part of thedescribed module can function either cooperatively or independently ofany other component forming a part of the module. Conversely, multiplemodules described herein can represent a single component within anactual device. Further, components within a module can be in a singledevice or distributed among multiple devices in a wired or wirelessmanner.

FIG. 1 illustrates an example Massive Multi-Input-Multi-Output (MIMO)(M-MIMO) transmitter 100. Example M-MIMO transmitter 100 is provided forthe purpose of illustration only and is not limiting of embodiments. Forthe purpose of simplification of presentation, certain components ofM-MIMO transmitter 100 are omitted in FIG. 1 as would be apparent to aperson of Skill in the art. Example M-MIMO transmitter 100 can belocated in a base station or a user equipment (UE).

As shown in FIG. 1, example M-MIMO transmitter 100 includes a pluralityof power amplifiers (PAs) 104.1, . . . , 104.N and a plurality ofantennas 106.1, . . . , 106.N. PAs 104.1, . . . , 104.N and antennas106.1, . . . , 106.N form a plurality of parallel transmit signal pathsof M-MIMO transmitter 100, each of which can be used independently ofthe others.

In an embodiment, M-MIMO transmitter 100 is used as part of a M-MIMOcommunication system to transmit information to a receiver (not shown inFIG. 1). For example, M-MIMO transmitter 100 can be located at a basestation of a multi--access communication network (e.g., cellularnetwork), and the receiver can be a user equipment (UE). As such, PAs104.1, . . . , 104.N are configured to receive respectively a pluralityof input signals 102.1, . . . , 102.N for transmission to the receiver.In an embodiment, input signals 102.1, . . . , 102.N are receivedsimultaneously or substantially simultaneously by PAs 104.1, . . . ,104.N. Input signals 1.02.1, . . . , 102.N can contain the same ordifferent information streams. PAs 104.1, . . . , 104.N are configuredto amplify respectively input signals 102.1, . . . , 102.N and toforward amplified signals 108.1, . . . , 108.N to antennas 106.1, . . ., 106.N respectively. In an embodiment, antennas 106.1, . . . , 106.Ntransmit amplified signals 108.1, . . . , 108.N simultaneously orsubstantially simultaneously to the receiver.

Because the plurality of transmit paths of M-MIMO transmitter 100 areparallel to each other, each of input signals 102.1, . . . , 102.N isprocessed (e.g., amplified and transmitted) in a single separatetransmit path. As a result, failure in a given transmit path (e.g.,failure of the PA and/or antenna of the transmit path) can cause thesignal being processed through it to be lost and not transmitted to thereceiver. This results in a reduction of the transmission spatial rate(number of parallel transmissions) from the transmitter to the receiverand a decrease in Signal-to-Noise Ratio (SNR) at the receiver.

FIG. 2 illustrates an example M-MIMO transmitter 200 using a hybridmatrix according to an embodiment. Example M-MIMO transmitter 200 isprovided for the purpose of illustration only and is not limiting ofembodiments. For the purpose of simplification of presentation, certaincomponents of M-MIMO transmitter 200 are omitted in FIG. 2 as would beapparent to a person of skill in the art. Example M-MIMO transmitter 200can be located in a base station or a UE.

Like example M-MIMO transmitter 100, example M-MIMO transmitter 200 alsoincludes a plurality of PAs 104.1, . . . , 104.N and a plurality ofantennas 106.1, . . . , 106.N. In addition, example M-MIMO transmitter200 includes a first N×N hybrid matrix 202 and a second N×N hybridmatrix 204.

Hybrid matrix 202 is configured to receive the plurality of inputsignals 102.1, . . . , 102.N and to generate a respective plurality ofhybrid signals 206.1, . . . , 206.N. In an embodiment, hybrid matrix 202is configured such that each of the plurality of hybrid signals 206.1, .. . , 206.N is a combination (e.g., linear combination) of the pluralityof input signals 102.1, . . . , 102.N. As such, each of hybrid signals206.1, . . . , 206.N includes a component from each of the plurality ofinput signals 102.1, . . . , 102.N.

PAs 104.1, . . . , 104.N are each configured to receive a respective oneof the plurality of hybrid signals 206.1, . . . , 206.N and to generatea respective one of a plurality of amplified hybrid signals 208.1, . . ., 208.N. Hybrid matrix 204 is configured to receive the plurality ofamplified hybrid signals 208.1, . . . , 208.N and to generate arespective plurality of output signals 210.1, . . . , 210.N. In anembodiment, hybrid matrix 204 is configured such that each of theplurality of output signals 210.1, . . . , 210.N is a combination (e.g.,linear combination) of the plurality of amplified hybrid signals 208.1,. . . , 208.N.

Output signals 210.1, . . . , 210.N each includes a component from eachof the plurality of input signals 102.1, . . . , 102.N. In anembodiment, hybrid matrix 202 and hybrid matrix 204 are configured tohave inverse transfer functions such that input signals 102.1, . . . ,102.N all experience a unity gain response by passing through hybridmatrix 202 and hybrid matrix 204. As such, the components from each ofthe plurality of input signals 102.1, . . . , 102.N appear with equalweight (1/N) in each Of output signals 210.1, . . . , 210.N. Outputsignals 210.1, . . . , 210.N are transmitted respectively by antennas106.1, . . . , 106.N to a receiver. In an embodiment, output signals210.1, . . . , 210.N are transmitted simultaneously or substantiallysimultaneously to the receiver.

Because each of output signals 210.1, . . . , 210.N is a combination ofthe plurality of input signals 102.1, . . . , 102.N, the failure of oneor more of PAs 104.1, . . . , 104.N and/or one or more of antennas106.1, . . . , 106.N does not cause a loss of a respective input signal102 and reduction in the transmission spatial rate from the transmitterto the receiver. For example, the failure of PA 104.1 does not result ininput signal 102.1 being not transmitted to the receiver as would be thecase in example M-MIMO transmitter 100. As such, example M-MIMOtransmitter 200 provides an architecture for protecting againstcomponent failures in a M-MIMO transmitter.

FIG. 3 illustrates an example hybrid matrix 300 according to anembodiment. Example hybrid matrix 300 is provided for the purpose ofillustration only and is not limiting of embodiments. Example hybridmatrix 300 can be used for hybrid matrix 202 or 204, for example, inexample M-MIMO transmitter 200.

For the purpose of illustration, example hybrid matrix 300 is shown asan 8×8 matrix that can receive 8 input signals 302 and generate 8 outputsignals 310. As would be understood by a person of skill in the art, anN×N hybrid matrix where N is any integer can be formed in a similarfashion as illustrated by example hybrid matrix 300.

In an embodiment, input signals 302 are input in an interleaved mannerinto hybrid matrix 300 and output signals 310 are produced in aninterleaved manner by hybrid matrix 300. In another embodiment, hybridmatrix 300 includes a first stage 304, a second stage 306, and a thirdstage 308 of 2×2 hybrid couplers. First stage 304 receives input signals302 and produces the inputs to second stage 306. Second stage 306 inturn produces the inputs to third stage 308, which generates outputsignals 310.

FIG. 4 illustrates an example 2×2 hybrid coupler 400 that can be used ina hybrid matrix, such as 8×8 hybrid matrix 300, according to anembodiment. Example hybrid coupler 400 is provided for the purpose ofillustration only and is not limiting of embodiments. As shown in FIG.4, example hybrid coupler 400 is configured to receive two input signals402.1 and 402.2 and to generate two output signals 404.1 and 404.2. Inan embodiment, output signal 404.1 is given by Y1=1/√2(−jX1−X2) andoutput signal 404.2 is given by Y2=1/√2(−X1−jX2), where X1 representsinput signal 402.1 and X2 represents input signal 402.2. In anotherembodiment, example hybrid coupler 400 is a 3-dB hybrid coupler.

In the above, embodiments where transmission from a M-MIMO transmitterto a receiver employed all of the N antennas of the M-MIMO transmitterwere provided. But, in some cases, transmitting using all of the Nantennas of the M-MIMO transmitter to a single receiver can be costlyand/or not necessary to achieve the desired performance at the receiver(e.g., where N is very large). In such cases, transmission may be moreefficient using only a subset M of the N antennas. Embodiments describedbelow provide methods and systems for determining such a subset of Mantennas.

FIG. 5 illustrates an example M-MIMO transmitter 500 using an antennaselection matrix according to an embodiment. Example M-MIMO transmitter500 is provided for the purpose of illustration only and is not limitingof embodiments. Example M-MIMO transmitter 500 can be located in a basestation or a UE. As shown in FIG. 5, example M-MIMO transmitter 500includes a plurality of PAs 104.1, . . . , 104.M, an antenna selectionmatrix 502, and a plurality of antennas 106.1, . . . , 106.N, where M isless than N. As would be understood by a person of skill in the artbased on the teachings herein, example M-MIMO transmitter 500 caninclude more than M PAs (e.g., N), and the embodiment of FIG. 5illustrates only those PAs (M of them) that are actively receiving oneof input signals 102.1, . . . , 102.M.

PAs 104.1, . . . , 104.M are configured to receive respectively aplurality of input signals 102.1, . . . , 102.M for transmission to areceiver. In an embodiment, input signals 102.1, . . . , 102.M arereceived simultaneously or substantially simultaneously by PAs 104.1, .. . , 104.M. Input signals 102.1, . . . , 102.M can contain the same ordifferent information streams. PAs 104.1, . . . , 104.M are configuredto amplify respectively input signals 102.1, . . . , 1.02.M and toforward the amplified signals 108.1, . . . , 108.M to antenna selectionmatrix 502.

In an embodiment, antenna selection matrix 502 is an M-input N-outputswitch matrix. Antenna selection matrix 502 is configured to couple eachof amplified signals 108.1, . . . , 108.M to a respective one ofantennas 10.6.1, . . . , 106.N. Because M is less than N, only M ofantennas 106.1, . . . , 106.N will have a signal to transmit, and N−Mantennas will not be used. In an embodiment, antenna selection matrix502 couples amplified signals 108.1, . . . , 108.M to antennas 106.1, .. . , 106.N randomly or according to a pre-determined order that ranksantennas 106.1, . . . , 106.N. In another embodiment, as described belowwith reference to FIG. 6, antenna selection matrix 502 couples amplifiedsignals 108.1, . . . , 108.M to antennas 106.1, . . . , 106.N based onan estimate of the channel from antennas 106.1, . . . , 106.N to theantennas of the receiver.

FIG. 6 illustrates another example M-MIMO transmitter 600 using anantennas selection matrix according to an embodiment. Example M-MIMOtransmitter 600 is provided for the purpose of illustration only and isnot limiting of embodiments. Example M-MIMO transmitter 600 can belocated in a base station or a UE. Like example M-MIMO transmitter 500described above, example M-MIMO transmitter 600 also includes aplurality of PAs 104.1, . . . , 104.M, an antenna selection matrix 502,and a plurality of antennas 106.1, . . . , 106.N, where M is less thanN. In addition, example M-MIMO transmitter 600 includes a measurementmodule 602. Measurement module 602 may include one or more processors toperform the functions described herein.

In an embodiment, measurement module 602 is configured to estimate achannel from the plurality of antennas 106.1, . . . , 106.N to aplurality (e.g., K) antennas at a receiver (K receiver antennas) towhich transmission of input signals 102.1, . . . , 102.M is intended. Inan embodiment, measurement module 602 is configured to receive, fromeach of the plurality of antennas 106.1, . . . , 106.N via respectivereceive signal paths 606.1, . . . , 606.N, K reference signalstransmitted respectively by the K receiver antennas. Using each of the Kreference signals, measurement module 602 can estimate a sub-channel hfrom each of the plurality of antennas 106.1, . . . , 106.N to arespective one of the K receiver antennas. The combination of thesub-channels for all K receiver antennas provides a full N×K channelestimate. In an embodiment, the reference signals include soundingreference signals (SRS) as defined by the Long Term Evolution (LTE)standard.

Based on the channel estimate, measurement module 602 is configured toselect a subset of size M of the plurality of antennas 106.1, . . . ,106.N to use for transmitting the plurality of amplified signals 108.1,. . . , 108.M to the receiver. In an embodiment, measurement module 602controls antenna selection matrix 502 using a control signal 604 tocouple the plurality of amplified signals 108.1, . . . , 108.M to theselected subset of the plurality of antennas 106.1, . . . , 106.N.Example embodiments for selecting the subset of the plurality ofantennas 106.1, . . . , 106.N, based on the channel estimate, aredescribed below. As would be understood by a person of skill in the artbased on the teachings herein, embodiments are not limited to theseexample embodiments.

In one embodiment, measurement module 602 is configured to select asubset that increases channel capacity from M-MIMO transmitter 600 tothe receiver. To determine this subset, measurement module 602 begins bychoosing a subset of size M from the plurality of antennas 106.1, . . ., 106.N. For the chosen subset, measurement module 602 forms, using theestimated channel, a sub-channel matrix h corresponding to the subset.The sub-channel matrix h includes the M×K row vectors of the channelestimate that correspond to the antennas of the subset. Measurementmodule 602 then forms a channel matrix A equal to a product of theconjugate transpose (Hermitian) of h by h, and performs a singular valuedecomposition (SVD) of A to determine a unitary matrix U, a diagonalmatrix D having positive diagonal elements S₁, . . . , S_(K), and anorthogonal matrix V. Then, measurement module 602 computes a function Sof the positive diagonal elements S₁, . . . , S_(K) of the diagonalmatrix B (S=f(S₁, . . . , S_(K))). In an embodiment, the function S is alinear sum of the positive diagonal elements (S=S₁+S_(K)). In anotherembodiment, the function S is a logarithmic sum of the positive diagonalelements (S=log₂(S₁)₊ . . . +log₂(S_(K))). In an embodiment, measurementmodule 602 repeats the above described process for all possible subsetsof size M (N choose M) of the plurality of antennas 106.1, . . . ,106.N. Then, measurement module 602 chooses the subset with the largestfunction S as the selected subset of the plurality of antennas 106.1, .. . , 106.N. In another embodiment, measurement module 602 prunes thelist of all possible subsets to eliminate quasi-duplicate subsets(subsets that are substantially similar due to their respective antennasbeing highly correlated), and performs the above process for only theremaining subsets.

In another embodiment, measurement module 602 is configured to select asubset that increases SNR at the receiver. To determine this subset,measurement, module 602 determines a matrix H Where each column vector hof H corresponds to a respective sub-channel between the plurality ofantennas 106.1, . . . , 106.N and a respective one of the plurality of(e.g., K) receiver antennas. Measurement module 602 then forms, for eachcolumn vector h of H a respective vector y by squaring each element ofthe column vector h. Measurement module 602 then forms a vector S byadding the vectors y (for K receiver antennas, K y vectors are added).Measurement module 602 then determines the M largest elements of thevector S, and selects M antennas of the plurality of antennas 106.1, . .. , 106.N that correspond to the M largest elements of vector S (theselected M antennas correspond to the indices of the M largest elementsof the vector S) as the selected subset of the plurality of antennas106.1, . . . , 106.N.

In another embodiment, example M-MIMO transmitter 600 receives feedbackfrom the receiver to which transmission of signals 102.1, . . . , 102.Mis intended regarding which subset of antennas to select.

FIG. 7 illustrates an example M-MIMO transmitter 700 using an antennaselection matrix and a hybrid matrix according to an embodiment. ExampleM-MIMO transmitter 700 is provided for the purpose of illustration onlyand is not limiting of embodiments. Example M-MIMO transmitter 700 canbe located in a base station or a UE. Like example M.-MIMO transmitter600 described above, example M-MIMO transmitter 700 also includes aplurality of PAs 104.1, . . . , 104.M, an antenna selection matrix 502,a measurement module 602, and a plurality of antennas 106.1, . . . ,106.N, where M is less than N. In addition, example M-MIMO transmitter700 includes two M×M hybrid matrices 702 and 704.

As shown in FIG. 7, hybrid matrix 702 is configured to receive theplurality of input signals 102.1, . . . , 102.M and to generate arespective plurality of hybrid signals 706.1, . . . , 706.M. In anembodiment, hybrid matrix 702 is configured such that each of theplurality of hybrid signals 706.1, . . . , 706.M is a combination (e.g.,linear combination) of the plurality of input signals 102.1, . . . ,102.M. As such, each of hybrid signals 706.1, . . . , 706.M includes acomponent from each of the plurality of input signals 102.1, . . . ,102.M.

PAs 104.1, . . . , 104.M are each configured to receive a respective oneof the plurality of hybrid signals 706.1, . . . , 706.M and to generatea respective one of a plurality of amplified hybrid signals 708.1, . . ., 708.M. Hybrid matrix 704 is configured to receive the plurality ofamplified hybrid signals 708.1, . . . , 708.M and to generate arespective plurality of output signals 710.1, . . . , 710.M. In anembodiment, hybrid matrix 704 is configured such that each of theplurality of output signals 710.1, . . . , 710.M is a combination (e.g.,linear combination) of the plurality of amplified hybrid signals 708.1,. . . , 708.M.

Output signals 710.1, . . . , 710.M each includes a component from eachof the plurality of input signals 102.1, . . . , 102.M. In anembodiment, hybrid matrix 702 and hybrid matrix 704 are configured tohave inverse transfer functions such that input signals 102.1, . . . ,102.M all experience a unity gain response by passing through hybridmatrix 702 and hybrid matrix 704. As such, the components from each ofthe plurality of input signals 102.1, . . . , 102.M appear with equalweight (1/M) in each of output signals 710.1, . . . , 710.M.

Antenna selection matrix 502 is an M-input N-output switch matrix. In anembodiment, antenna selection matrix 502 is configured to receive outputsignals 710.1, . . . , 710.M and to couple each of output signals 710.1,. . . , 710.M to respective one of a subset of size M of antennas 106.1,. . . , 106.N. Because M is less than N, only M of antennas 106.1, . . ., 106.N will have a signal to transmit, and NM antennas will not beused.

In an embodiment, antenna selection matrix 502 couples output signals710.1, . . . , 710.M to antennas 106.1, . . . , 106.N in accordance withcontrol signal 604 from measurement module 602. In another embodiment,as described above with reference to FIG. 6, measurement module 602 isconfigured to select the subset of size M of the plurality of antennas106.1, . . . , 106.N based on an estimate of a channel from theplurality of antennas 106.1, . . . , 106.N to a plurality of (e.g., K)antennas at a receiver (K receiver antennas) to which transmission ofinput signals 102.1, . . . , 102.M is intended.

FIG. 8 illustrates an example process 800 for M-MIMO antenna selectionaccording to an embodiment. Example process 800 can be performed by anM-MIMO transmitter, such as example M-MIMO transmitters 600 and 700described above, for example.

As shown in FIG. 8, process 800 begins in step 802, which includesestimating a channel from a plurality of first antennas to a pluralityof second antennas. In an embodiment, step 802 can be performed by ameasurement module, such as measurement module 602 described above withreference to FIG. 6. In an embodiment, the plurality of first antennasare located at a base station and the plurality of second antennas arelocated at a UE. In another embodiment, the plurality of first antennasare located at the UE and the plurality of second antennas are locatedat the base station.

In an embodiment, step 802 farther includes receiving, from each of theplurality of first antennas, a plurality of reference signalstransmitted respectively by the plurality of second antennas; andestimating the channel using the plurality of reference signals. Theplurality of reference signals can include sounding reference signals(SRS).

Next, process 800 proceeds to step 804, which includes selecting, basedon the estimated channel, a subset of the plurality of first antennas.In an embodiment, step 804 can be performed by a measurement module,such as measurement module 602 described above with reference to FIG. 6.In an embodiment, the subset of the plurality of first antennas is usedto transmit a plurality of signals to the plurality of second antennas,and step 804 further includes selecting the subset of the plurality offirst antennas that increases channel capacity from the plurality offirst antennas to the plurality of second antennas and/or that increasesSNR at the receiver. In another embodiment, step 804 further includesperforming a process as described below with reference to FIGS. 9 and 10to select the subset of the plurality of first antennas.

Process 800 terminates in step 806, which includes transmitting aplurality of signals using the selected subset of the plurality of firstantennas to the plurality of second antennas. In an embodiment, step 806can be performed by an antenna selection matrix, such as antennaselection matrix 502 described above with reference to FIG. 5, and thesubset of the plurality of first antennas selected in step 804.

FIG. 9 illustrates another example process 900 for M-MIMO antennaselection according to an embodiment. Example process 900 is providedfor the purpose of illustration only and is not limiting of embodiments.Example process 900 can be performed by an M-MIMO transmitter having aplurality of antennas, such as example M-MIMO transmitters 600 and 700described above, for example. More specifically, process 900 can beperformed by a measurement module, such as measurement module 602described above with reference to FIG. 6, to select a subset of antennasfrom the plurality of antennas once a channel estimate has beendetermined. Process 900 assumes for the purpose of illustration that thenumber of the plurality of antennas is equal to N, that the size of theselected subset is equal to M, and that the intended receiver has Kantennas.

As shown in FIG. 9, process 900 begins in step 902, which includeschoosing a subset of M antennas from a total of N antennas. For thechosen subset, step 904 includes forming, using the channel estimate(N×K channel estimate), a sub-channel matrix h corresponding to thesubset. The sub-channel matrix h includes the M×K row vectors of thechannel estimate that correspond to the antennas of the subset.

Then, in step 906, process 900 includes forming a channel matrix A equalto a product of the conjugate transpose (Hermitian) of h by h.Subsequently, in step 908, a singular value decomposition (SVD) of A isperformed to determine a unitary matrix U, a diagonal matrix D havingpositive diagonal elements S₁, . . . , S_(K), and an orthogonal matrixV.

Then, in step 910, process 900 includes computing a function S of thepositive diagonal elements S₁, . . . , S_(K) of the diagonal matrix D(S=f(S₁, . . . , S_(K))). In an embodiment, the function S is a linearsum of the positive diagonal elements (S=S₁ ₊ . . . S_(K)). In anotherembodiment, the function S is a logarithmic sum of the positive diagonalelements (S=log₂(S₁)₊ . . . +log₂(S_(K))).

At step 912, process 900 includes determining whether all N choose Mcombinations (all subsets of size M) have been tested. If not, process900 returns to step 902 to choose a new subset of M antennas that hasnot yet been tested. Otherwise, process 900 proceeds to step 914, Whichincludes choosing the subset with the largest function S as the selectedsubset of the plurality of antennas. In an embodiment, this subsetmaximizes channel capacity from the M-MIMO transmitter to the receiver.

FIG. 10 illustrates another example process for M-MIMO antenna selectionaccording to an embodiment. Example process 1000 is provided for thepurpose of illustration only and is not limiting of embodiments. Exampleprocess 1000 can be performed by an M-MIMO transmitter having aplurality of antennas, such as example M-MIMO transmitters 600 and 700described above, for example. More specifically, process 1000 can beperformed by a measurement module, such as measurement module 602described above with reference to FIG. 5, to select a subset of antennasfrom the plurality of antennas once a channel estimate has beendetermined. Process 1000 assumes for the purpose of illustration thatthe number of the plurality of antennas is equal to N, that the size ofthe selected subset is equal to M, and that the intended receiver has Kantennas.

As shown in FIG. 10, process 1000 begins in step 1002, which includesdetermining a matrix H where each column vector h of H corresponds to arespective sub--channel between the plurality of antennas and arespective one of the plurality of (e.g., K) receiver antennas (eachcolumn vector h has N elements). Subsequently, in step 1004, process1000 includes forming, fur each column vector h of H, a respectivevector y by squaring each element of the column vector h (each vector yhas N elements). Then, in step 1006, process 1000 includes forming avector S by adding the vectors y obtained in step 1004 (the vector S hasN elements). For K receiver antennas. S is the sum of K vectors y.

In step 1008, the largest M elements (out of N) of S are determined.Finally, in step 1010, the M antennas (of the plurality of N antennas)with indices corresponding to the largest M elements of S are selectedas the selected subset of the plurality of antennas. In an embodiment,this subset maximizes SNR at the receiver.

Embodiments have been described above with the aid of functionalbuilding blocks illustrating the implementation of specified functionsand relationships thereof. The boundaries of these functional buildingblocks have been arbitrarily defined herein for the convenience of thedescription. Alternate boundaries can be defined so long as thespecified functions and relationships thereof are appropriatelyperformed.

The foregoing description of the specific embodiments will so hillyreveal the general nature of the disclosure that others can, by applyingknowledge within the skill of the art, readily modify and/or adapt forvarious applications such specific embodiments, without undueexperimentation, without departing from the general concept of thepresent disclosure. Therefore, such adaptations and modifications areintended to be within the meaning and range of equivalents of thedisclosed embodiments, based on the teaching and guidance presentedherein. It is to be understood that the phraseology or terminologyherein is for the purpose of description and not of limitation, suchthat the terminology or phraseology of the present specification is tohe interpreted by the skilled artisan in light of the teachings andguidance.

The breadth and scope of embodiments of the present disclosure shouldnot be limited by any of the above-described exemplary embodiments, butshould be defined only in accordance with the following claims and theirequivalents.

What is claimed is:
 1. A Multi-Input-Multi-Output (MIMO) antenna system, comprising: a plurality of first antennas; and a measurement module, coupled to the plurality of first antennas, configured to estimate a channel from the plurality of first antennas to a plurality of second antennas and to select a subset of the plurality of first antennas, based on the estimated channel, for transmitting a plurality of signals to the plurality of second antennas.
 2. The MIMO antenna system of claim 1, wherein the plurality of first antennas are located at a base station, and the plurality of second antennas are located at a user equipment (UE).
 3. The MIMO antenna system of claim 1, further comprising: an antenna selection matrix configured to receive the plurality of signals for transmission, wherein the measurement module is further configured to control the antenna selection matrix to couple the plurality of signals to the selected subset of the plurality of first antennas.
 4. The MIMO antenna system of claim 3, wherein the antenna selection matrix includes an M-input N-output switch matrix, where M is equal to the number of the plurality of signals and N is equal to the number of the plurality of first antennas.
 5. The MIMO antenna system of claim 4, further comprising: a first M-input M-output hybrid matrix configured to receive a plurality of input signals and to generate a respective plurality of hybrid signals, wherein each of the plurality of hybrid signals is a combination of the plurality of input signals; a plurality of power amplifiers (PAs) each configured to receive a respective one of the plurality of hybrid signals and to generate a respective one of a plurality of amplified hybrid signals; and a second M-input M-output hybrid matrix configured to receive the plurality of amplified hybrid signals and to generate the plurality of signals, wherein each of the plurality of signals is a combination of the plurality of amplified hybrid signals.
 6. The MIMO antenna system of claim 1, wherein the measurement module is further configured to receive, from at least one of the plurality of first antennas, a plurality of reference signals transmitted respectively by the plurality of second antennas and to estimate the channel using the plurality of reference signals.
 7. The MIMO antenna system of claim 6 wherein the plurality of reference signals include sounding reference signals (SRS).
 8. The MIMO antenna system of claim 1, wherein the size of the selected subset of the plurality of first antennas is equal to M, and wherein the measurement module is further configured, for each subset of size M of the plurality of first antennas: form, using the estimated channel, a sub-channel matrix h corresponding to a sub-channel between the subset and the plurality of second antennas; form a channel matrix A equal to a product of the conjugate transpose of h by h; perform a singular value decomposition (SVD) of A to determine a unitary matrix U, a diagonal matrix D having positive diagonal elements, and an orthogonal matrix V; and compute a function of the positive diagonal elements of the diagonal matrix D.
 9. The MEMO antenna system of claim 8, wherein the function is a sum of the positive diagonal elements of the diagonal matrix D.
 10. The MIMO antenna system of claim 8, wherein the measurement module is further configured to choose the subset with the largest function as the selected subset of the plurality of first antennas.
 11. The MIMO antenna system of claim 1, wherein the size of the selected subset of the plurality of first antennas is equal to M, and wherein the measurement module is further configured to: determine a matrix H wherein each column vector h of H corresponds to a respective sub-channel between the plurality of first antennas and a respective one of the plurality of second antennas; form, for each column vector h of H, a respective vector y by squaring each element of the column vector h; form a vector S by adding the vectors y; determine the M largest elements of the vector S; and select M antennas of the plurality of first antennas that correspond to the M largest elements of vector S as the selected subset of the plurality of first antennas.
 12. A method for Massive Multi-Input-Multi-Output (MIMO) (M-MIMO) antenna selection, comprising: estimating a channel from a plurality of first antennas to a plurality of second antennas; selecting, based on the estimated channel, a subset of the plurality of first antennas; and transmitting a plurality of signals to the plurality of second antennas using the subset of the plurality of first antennas.
 13. The method of claim 12, *herein the plurality of first antennas are located at a base station, and the plurality of second antennas arc located at a user equipment (UE).
 14. The method of claim 12, further comprising: receiving, from at least one of the plurality of first antennas, a plurality of reference signals transmitted respectively by the plurality of second antennas; and estimating the channel using the plurality of reference signals.
 15. The method of claim 12, wherein the size of the selected subset of the plurality of first antennas is equal to M, and wherein the method further comprises, for each subset of size M of the plurality of first antennas: forming, using the estimated channel, a subset channel matrix h corresponding to a sub-channel between the subset and the plurality of second antennas; forming a channel matrix A equal to a product of the conjugate transpose of h by h; performing a singular value decomposition (SVD) of A to determine a unitary matrix U, a diagonal matrix D having positive diagonal elements, and an orthogonal matrix V; and computing a function of the positive diagonal elements of the diagonal matrix D.
 16. The method of claim 15, wherein the function is a sum of the positive diagonal elements of the diagonal matrix D.
 17. The method of claim 15, further comprising: choosing the subset with the largest function as the selected subset of the plurality of first antennas.
 18. The method of claim 12, wherein the size of the selected subset of the plurality of first antennas is equal to M, and wherein the method further comprises: determining a matrix H wherein each column vector h of H corresponds to a respective sub-channel between the plurality of first antennas and a respective one of the plurality of second antennas; forming, for each column vector h of H, a respective vector y by squaring each element of the column vector h; forming a vector S by adding the vectors y; determining the M largest elements of the vector S; and selecting M antennas of the plurality of first antennas that correspond to the M largest elements of vector S as the selected subset of the plurality of first antennas.
 19. A Multi-Input-Multi-Output (MIMO) antenna system, comprising: a first hybrid matrix configured to receive a plurality of input signals and to generate a respective plurality of hybrid signals, wherein each of the plurality of hybrid signals is a combination of the plurality of input signals; a plurality of power amplifiers (PAs) each configured to receive a respective one of the plurality of hybrid signals and to generate a respective one of a plurality of amplified hybrid signals; a second hybrid matrix configured to receive the plurality of amplified hybrid signals and to generate a respective plurality of output signals, wherein each of the plurality of output signals is a combination of the plurality of amplified hybrid signals; and a plurality of antennas each configured to transmit a respective one of the plurality of output signals.
 20. The MIMO antenna system of claim 19, wherein the first hybrid matrix and the second hybrid matrix have inverse transfer functions. 